The Long Term Evolution (LTE) is currently being standardized by 3GPP. As compared to earlier radio technologies, it provides higher peak data rates up to 300 Mbit/s, improved spectrum efficiency and reduced radio access delays. One key requirement in development of LTE has been spectrum flexibility; LTE can be operated in different spectrum allocations from 1.4 to 20 MHz and in paired or unpaired spectrum. With the paired spectrum, the Frequency Division Duplex (FDD) mode uses different carrier frequencies for downlink and uplink, whereas with the unpaired spectrum, the Time Division Duplex (TDD) mode uses a single carrier frequency and separation of downlink and uplink in time. Regardless of this fundamental difference, the basic design principle in LTE has been that FDD and TDD should be as similar as possible.
Similar to FDD, the TDD radio frame consists of 10 subframes, each having the length of 1 ms. In a radio frame, a subframe can be either an uplink (UL), downlink (DL) or a special subframe.
The expression “downlink” is in the present context used to specify the transmission from the base station to the user equipment, while the expression “uplink” is used to denote the transmission from the user equipment to the base station.
A general principle is that the subframes 0 and 5 may be downlink subframes, subframe 2 may be an uplink subframe, and subframe 1 may be a special subframe. As can be seen from FIG. 1, the special frame comprises a downlink part (DwPTS), a guard period (GP) and an uplink part (UpPTS). The downlink part of the special frame can be considered as a normal downlink subframe for data and control but with a reduced number of data symbols. The guard period comprises a number of idle symbols when nobody is transmitting. Finally, the uplink part of the special frame is considerably shorter than the downlink part and is primarily used for sounding and random access preamble transmission, rather than for user data transmission.
In FIG. 1, the downlink and uplink allocation in subframes within a radio frame is illustrated for Configuration 1. The direction of the arrow in each subframe indicates uplink or downlink respectively. The subframes without arrows are guard periods.
3GPP has defined 7 downlink-uplink configurations, which are listed in Table 1. The configurations cover a wide range of allocations from the downlink focused 9:1 configuration 5, to the uplink focused 2:3 configuration 0. In the table, the DL:UL ratio refers to how the downlink and uplink periods are repeated. For example, with Configuration 1 having DL:UL ratio 3:2, there are three downlink or special subframes followed by two uplink subframes.
TABLE 1ConfigurationDL:UL02:313:224:137:348:259:163:3:2:2
The selection of TDD uplink-downlink configuration is done based on known traffic characteristics and asymmetries of the network. The selection should be done in such a way that the available spectrum is utilized most efficiently. However, if downlink and uplink co-exists in the neighbouring cells, significant interference can occur. A radio base station (RBS) receives interference from other base stations and a user equipment (UE) receives interference from other user equipments, see FIG. 2. Interference is in the present context anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal. Interference may typically but not always be distinguished from noise, for example white thermal noise.
Interference may be measured by measuring a signal-to-interference ratio (S/I or SIR), also known as the carrier-to-interference ratio (C/I, CIR), which is the quotient between the average received modulated carrier power, S or C respectively, and the average received co-channel interference power I, i.e. cross-talk, from other transmitters than the useful i.e. information carrying signal.
In order to avoid base station-to-base station and user equipment-to-user equipment interference, the same TDD uplink-downlink allocation is commonly used in the entire wireless network, or at least in an entire geographical region. Therefore, an adaptation to the instantaneous load in a particular cell is seldom possible. However, in many situations one cell can be uplink capacity limited whereas another cell is downlink limited. Thus, having a dynamic TDD configuration would improve the performance of the network. In addition, extreme uplink-downlink configurations could be utilized more efficiently because the entire region would not need to follow the same configuration.
From the standard point of view, it is possible to have different uplink-downlink TDD configurations in the neighbouring cells in a network supporting TDD access mode. However, in practice this may be difficult without a dedicated solution for interference management.
According to previously known solutions for base station-to-base station interference management, the neighbouring cells within a network can have different TDD uplink-downlink configurations. The Time to Transmit Intervals (TTIs) which have different link direction (uplink/downlink) in neighbour cells than in the own cell are called “flexible” TTIs. This term is used also in this document.
In the previously known interference management solution, the base station-to-base station interference (BBI) between neighbour cells is firstly estimated. If this interference is below a given threshold, preferably close to 0 dB, the data transmission is possible in the flexible TTI even the uplink-downlink direction is different. For neighbour cells not satisfying this condition and having different uplink/downlink direction, flexible TTIs are not used for data transmission.
An example of dynamic TDD configuration solution can be seen in FIG. 3. In this Figure, it can be seen that cells #6 and #7 have uplink in the flexible TTI, whilst their neighbour #12 has downlink. Data transmissions are possible in the flexible TTIs since the BBI interference between these respective sites is below a given threshold, probably very close to 0 dB. For neighbour cells which do not satisfy the BBI condition, user equipments are not scheduled on the flexible TTI. This is the case for cells #8 and #13, #14 in FIG. 3.
However, even if BBI is close to 0 dB, there is no guarantee that user equipment-to-user equipment interference or mobile-to-mobile (MMI) interference is avoided. This can be seen in FIG. 2, where the channel BB12 is not necessarily correlated with the channel MM12. It is thus a need to protecting user equipment from MMI.